This invention relates broadly to a mechanical press, and particularly to a press brake having a linear actuator.
More particularly, the present invention relates to a press brake comprising a displacement sensor having an electric signal output and adapted to detect the position of a ram plate.
In general, a press brake used for bending sheet metal or the like is required to control the position of its ram plate with very high accuracy.
For example, as is shown in a side view of FIGS. 2a and 2b (FIG. 1 shows the present invention and will therefore be described afterward), a plate such as a metal plate 20 or the like is interposed between an upper die 11 and a lower die 12 and pressed so that the plate is usually bent at an angle of 90.degree.. However, since the accuracy required for the press brake is so high, bent plates with errors within +0.5 with respect to a target angle of 90.degree. are rejected.
Therefore, the control of the pressing position of the upper die 11 is required to be so accurate that the errors must usually remain within 1/100 mm, approximately. Besides, a large amount of power is required for bending a wide plate member. To this end, the usual practice is that a pressure is applied to both ends of a ram plate 30 by a hydraulic cylinder 40 (see FIG. 3). Of course, the upper die 11 can be held stationary and the lower die 12 can be pushed up. Right and left cylinders 41 and 42 are required to be synchroneously driven with high accuracy. However, when a workpiece is interposed between the upper die 11 and the lower die 12, it is often displaced leftward or rightward, and when displaced, a one-sided load is created. Since the load is not equally applied to the right and left cylinders 41 and 42, it is very difficult for them to be driven in synchronism.
The plate which is the workpiece is not always required to be bent at an equal angle. Sometimes it is required that the bending angle of the left end of the plate be different from that of the right end as shown in the perspective view of FIG. 4. In this case, the pressure must be applied to the ram 30 held in an inclined state as shown in FIG. 5 with synchroneous drive of the cylinders 41 and 42.
Such control is generally obtainable by detecting the movement (displacement) of the ram plate 30 by a linear scale and feeding it back to a servo valve or a proportional control valve for controlling the cylinders 41 and 42. If this is carried out according to an analog system, a satisfactory control can be obtained. However, it is not easy to obtain an accuracy of 1/100 mm over the whole stroke. On the other hand, if a digital system is adopted, the detection of the displacement and the control of the cylinder are usually made by a microcomputer. In this case, if the displacement is to be read to the limit of discrimination of the linear scale by computer, it takes too much time for reading and controlling, and the speed of action of the cylinder is proportionately lowered. If some data from the linear scale is intentionally not read in order to save time, the result is the same as if the discrimination of the linear scale was made rough. This means that the synchroneous drive of the right and left cylinders becomes insufficient.
There is also known a press brake having a mechanical feed mechanism as shown in the front view of FIG. 6. In this conventional press brake, a plunger of a 4-port switch valve 60 is pushed by a linear actuator 50 (e.g., a combination of a rotary encoder, a servo motor, ball screw, etc.) for switching the port, thereby to actuate a hydraulic cylinder 40 to follow the ram plate 30. In other words, the ram plate 20 is caused to follow the movement of the linear actuator 50 by using a mechanical feed back system.
However, this system has the shortcoming that it is very difficult to switch the port by a minor displacement of the plunger. In order to respond to the minor displacement, the mechanical accuracy of the spool and port must be extremely good or the movement of the linear actuator must be increased according to the principle of a lever. However, if a lever mechanism is introduced into such an arrangement, it becomes very unstable in the areas of such things as initial adjustment, change due to age, or the like.
In the case of a mechanical feed back servo control, the feed back loop gain cannot be adjusted during the operation test. Even if a mechanical system is taken into consideration, it becomes complicated and always has such problems as change due to age or the like. When a plate member is bent by actually applying pressure thereto, since the jaw portion of a frame 6 shown in a side view of FIG. 7 is spread, the cylinder 40 must be further pushed down to that extent. This means that in order to accurately control the actual distance D between the upper die 11 and the lower die 12, it is necessary to know the amount of spread of the frame 6 as data beforehand. This always causes difficulties in control.
The present invention has been accomplished in order to overcome the above-described problems.